In the field of packaging integrated circuits, there is a constant drive to make the packaging more compact, so that more chips per unit of area can be fitted within a given volume.
Various schemes to stack chips vertically have been proposed, but that suffer from various drawbacks. In the particular case of ball grid array contacts, which are preferred in many applications because of superior electrical performance, the vertical height of the chips is an issue, since excessive vertical height must be compensated for by large-diameter balls or by expensive alternatives such as putting down spacer layers to build up the height.
Using a cavity in a printed circuit board has the drawback that the cost of forming a cavity can be excessive in a cost-conscious field such as packaging. Special-configuration boards that are manufactured in limited numbers also suffer from an associated high inventory cost.
Passing signals between vertically separated chips also presents a challenge to achieve reliably and economically.